The invention relates to a control valve used for shifting speed range in an automatic transmission.
An automatic transmission has a plurality of gear trains arranged in parallel, one of which is automatically selected to give a speed reduction ratio appropriate for the driving condition. In order to select a desirable gear train and achieve automatic shifting, a hydraulic clutch is used, which is controlled by means of a control valve by controlling the pressure of the operational fluid to be supplied thereinto or drained therefrom. There have been many automatic transmissions using such control valves in the past, as disclosed for example in Japanese Patent Publication No. 52 (1977)-21131.
In automatic shifting in the automatic transmission, the hydraulic pressure in the hydraulic chamber of the clutch which has been in operational engagement (hereinafter referred to as "pre-shift clutch") is relieved by exhausting fluid to a drain, and the operational fluid having a predetermined line pressure is supplied from a pressure line into the hydraulic chamber of the clutch for the gear train set up for the desired speed range (hereinafter referred to as "post-shift clutch"). The operational fluid in the hydraulic chamber of the pre-shift clutch is allowed to escape to the drain by a hydraulic relief valve which is operated by means of the hydraulic pressure supplied from the hydraulic chamber of the post-shift clutch so that the pressure supply from the pressure line into the post-shift clutch chamber may be simultaneously timing with the drainage from the pre-shift clutch chamber.
In the case when the speed range set by the pre-shift clutch is an intermediate speed range, there can be two modes of speed range shifting, i.e., a down-shift to give a greater speed reduction ratio and an up-shift to give a smaller speed reduction ration. Consequently two hydraulic relief valves, one for the down-shift and another for the up-shift, have been conventionally used in an automatic transmission.
The use of two relief valves increases not only a cost to manufacture them but also the space to install them.